Don't even think about attempting this on a system you don't own and have full permission to do so. You WILL get caught. Every one of these methods leaves traces behind that make it very obvious that someone hacked the Administrator account.

With a limited account you are unable to install any new program on your computer. But there are still two options for you to regain full access to your computer: one is reset your lost admin password; another option is promote your limited account to an administrator.

These can be done easily. To get started, you need to use an alternate PC to create a PCUnlocker Live CD, then boot up your locked PC from the CD and it enables you to either reset any user password or promote a limited account to administrator.